GaN-based compound semiconductors, such as gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN), are spotlighted as materials for a blue-light-emitting diode (LED) and a laser diode (LD). Furthermore, development for the application of GaN-based compound semiconductors for electronic devices has been started utilizing their features of high heat resistance and environment friendliness.
The bulk crystal growth of GaN-based compound semiconductors is difficult; therefore, a GaN substrate that could be adapted to practical use has not yet been obtained. A substrate for GaN growth widely put to practical use today is sapphire, and a method for the epitaxial growth of GaN on a single-crystal sapphire substrate by metal-organic vapor phase epitaxy (MOVPE) technique and the like is commonly used.
Since the lattice constant of a sapphire substrate differs from that of GaN, a continuous GaN single crystalline film cannot be grown directly on the sapphire substrate. Therefore, a process wherein the strain of the lattice is relieved by a buffer layer of AlN or GaN grown on the sapphire substrate at a low temperature, and then GaN is grown thereon has been proposed (Japanese Patent Laid-Open No. 63-188983). The use of a nitride layer grown at a low temperature as a buffer layer has enabled the single-crystalline epitaxial growth of GaN. However, even this method cannot compensate the lattice-mismatch between the substrate and the crystal, and the GaN film has numerous defects. These numerous defects are considered to interfere with the manufacture of a GaN-base LD. In recent years, as a method for reducing the density of the defects caused by the difference in lattice constants between sapphire and GaN, crystal growth techniques, such as ELO (Appl. Phys. Lett. 71 (18) 2638 (1997)), FIELO (Jpn. J. Appl. Phys. 38, L184 (1999)), and pendeo epitaxy (MRS Internet J. Nitride Semicond. Res. 4S1, G3.38 (1999)) have been reported, and GaN epitaxial wafers having a drastically high crystalline quality have been obtained.